This request is for the continuation of a project aimed at contributing to our understanding of enzyme catalysis by providing information on the steric course of a number of enzyme reactions at prochiral and pro-prochiral centers. The work does, for the most part, involve the use of isolated enzymes and substrates labeled stereospecifically with tritium and/or deuterium followed, if necessary, by stereospecific degradations of the products. Enzyme reactions to be investigated are a number of pyridoxal phosphate-catalyzed alpha, Beta- elimination and elimination-addition reactions of amino acids, the transfer of methyl groups in biological C-, N-, O- and S-methylation reactions, the isoprenylation of aromatic ring systems and some enzyme reaction in which methyl groups are transformed into methylene groups or vice versa (citrate synthetase, isopentenyl pyrophosphate isomerase, thymidylate synthetase).